1. Field of the Invention
The present invention relates to an optical scanner.
2. Description of the Related Art
In various kinds of general known optical scanners relative to an optical printer, etc., a light beam deflected at an equal angular velocity is converged as a light spot on a scanned face to scan the scanned face. An f.sup..theta. lens is generally known as an optical system for converging the deflected light beam as the light spot on the scanned face. However, an image forming mirror having a linearity correcting function is recently used instead of the f.sup..theta. lens as a proposal. For example, such a proposal is shown in Japanese Patent Application Laying Open (KOKAI) No. 1-200221.
In a recent optical scanner, a high density record is required to improve the quality of a written image. To do this, it is necessary that no diameter of a light spot for scanning the scanned face is greatly changed as the height of an image is increased. The diameter of the light spot is influenced by loci of an image forming point in a main scan-corresponding direction and a cross scan-corresponding direction, i.e., field curvatures in these directions in an image forming system for converging the deflected light as a light spot. The main scan-corresponding direction is set to a direction corresponding and parallel to a main scanning direction on a virtual optical path on which an optical path from a light source to the scanned face is linearly developed. The cross scan-corresponding direction is set to a direction corresponding and parallel to a cross scanning direction on this virtual optical path.
It is sufficient to set the field curvatures of the image forming system to zero in both the main scan-corresponding direction and the cross scan-corresponding direction so as to restrain the change in diameter of the light spot. However, it is practically impossible to set the field curvatures of the image forming system to zero. In reality, the field curvatures are considerably caused in the main scan-corresponding direction and the cross scan-corresponding direction. The change in diameter of the light spot caused by the field curvature in the main scan-corresponding direction can be corrected by electrically controlling a time for writing one dot to such an extent that no practical problems about this change are caused. In contrast to this, it is difficult to make such an electric correction in the case of the change in diameter of the light spot in the cross scan-corresponding direction. Accordingly, when there is field curvature of the image forming system in the cross scan-corresponding direction, the diameter of the light spot in the cross scan-corresponding direction is changed in accordance with the height of an image.
Further, when there is a so-called inclination of a deflecting face in an optical deflector for deflecting a light beam at an equal angular velocity, the position of a main scanning line scanned by a light spot is changed in the cross scanning direction so that so-called pitch irregularities of the scanning line are caused. The diameter of the light spot is changed and the pitch irregularities are caused even when an image forming mirror is used as the image forming system.
When the image forming mirror is used, the light beam formed as an image by the image forming mirror is reflected onto the same side as a light beam incident to the image forming mirror. Therefore, it is necessary to dispose a means for separating optical paths of the incident and reflected light beams from each other. The main scanning line is curved by the separation of the optical paths. This curve in a main scanning line is a problem peculiar to usage of the image forming mirror.